Under the Romans, Ostia Antica reached a peak of some 75,000 inhabitants in the 2nd and 3rd centuries AD. The city became an
episcopal see as part of the
Diocesi of Rome as early as the 3rd century.
St. Augustine passed through in the late 4th century; his mother,
St. Monica, died here in 387 in a house property of the Diocesi of Rome. The cathedral (
titulus) of
Santa Aurea was later erected over her tomb. In 414 the poet
Rutilius Namatianus reported a lack of maintenance of the city; it is traditionally thought that Ostia began a slow decline in the time of
Constantine I, but data from recent excavations put the date of decline later. The city contained 26 operating baths in the 4th century and there is plenty of evidence of repairs on public buildings and the construction of new edifices. There was decay in some areas, but this late period was primarily one of transition; formerly filled with workers employed in collecting, storing and moving huge amounts of grain, oil, wine and other products to feed Rome, Ostia now assumed the character of a seaside resort. The city remained prosperous into the 5th century, as it was the seat of the
Praefectus annonae. There was expansion beyond the western and southern walls in the area of the Porta Marina. As the centuries passed, Ostia fell into ruin but continued to provide maritime access for visitors to Rome.
Saracen pirates were a frequent concern; the naval
Battle of Ostia was fought off the coast in 849.
Pope Gregory IV fortified the existing borough and it was rechristened
Gregoriopoli. By this time, the shifting course of the
Tiber had landlocked the ancient port, and the town was mainly a shelter for the workers of the nearby salt mills. In the late 15th century, the bishop
Giuliano della Rovere (later
Pope Julius II) commissioned the rebuilding of the main church and town walls under the direction of the architect
Baccio Pontelli. The Castle of Julius II, also built at this time, remains the most striking feature of modern Ostia. The castle was abandoned after a flood in 1587 inundated its moat and turned the surrounding area into a marsh. The castle and the town were restored again in the 20th century. == Geography ==